Several approaches have been considered to provide low-cost, high-performance, and high-density transmitters for Wavelength Division Multiplexing (WDM) applications. One approach is a “monolithic” approach, where a plurality of semiconductor lasers, modulators, and multiplexers (MUXs) are fabricated and packaged on a single chip to produce a WDM transmitter system. This approach can reduce the size and cost of WDM transmitter systems and increase port density. The temperature of the semiconductor lasers also can be controlled for proper operation by sharing a common cooler, which reduces power consumption. However, since the monolithic approach combines many optical components into one system, this approach has a typically low yield.
Another approach to provide WDM transmitter systems is the “hybrid” integration approach, where a plurality of laser dice may be integrated on a planar lightwave circuit (PLC) platform and coupled to a MUX to produce a WDM laser transmitter. The laser dice and the MUX may be separately fabricated using different suitable materials and then coupled together. The hybrid integration approach may provide similar advantages as the monolithic approach, reduce the quantity of integrated components, and improve yield.
Controlling the wavelength of individual lasers may be challenging using either approach. The laser dice of the WDM laser transmitter each may transmit light at a different wavelength, which may vary from the desired or design wavelength by some offset due to the fabrication process and ambient temperature. To rectify the offset of each laser, a plurality of heaters may be individually coupled to at least some of the laser dice and used to heat the lasers to thermally tune the wavelengths. However, due to the close proximity of the lasers, which may be less than one millimeter (mm), heating the individual lasers may result in undesired thermal crosstalk, e.g., heat transfer between adjacent lasers. Thermal crosstalk between the lasers may reduce the wavelength tuning window and introduce additional heat to the lasers, which may make it more difficult to thermally control the wavelengths. Thermal crosstalk also may require more complicated protection schemes against heating failures.